1. Field of the Invention
The present invention relates to a spin-coating method for spin-coating a substrate surface in the shape of a square or the like with a resist film, a resist bottom anti-reflective coating (BARC), a resist top anti-reflective layer (TARL), a resist top protective film, a conductive film or the like (hereafter may be collectively referred to as a resist film) by a specific method. In particular, the present invention relates to a determination method for a resist spin-coating condition in order to make the coating thickness of the resist film further uniform, and a resist spin-coating method, as well as a photomask blank, etc., including a resist film, a resist bottom anti-reflective coating, a resist top anti-reflective layer, a resist top protective film, a conductive film or the like, or a combination thereof, formed by this method.
2. Description of the Related Art
In spin coating of resist solution, a commonly used spin-coating device has a basic configuration shown in FIG. 1. In FIG. 1, the spin-coating device comprises a nozzle 1 for supplying resist solution, a chuck 2 for chucking a substrate 3 and attached onto a cup 4, and a motor 5 for rotating the chuck 2 together with the substrate 3. As an example of the spin-coating method using such a spin-coating device, the following resist-coating method has been proposed (Japanese Patent Publication No. 4-29215). This resist-coating method aims to make the coating film thickness uniform, especially regarding a square-like (square or rectangular) substrate. This resist-coating method is composed of a uniforming (evening or flatting) step and a subsequent drying step. In the uniforming step, a substrate is rotated by selecting a setting rotation speed corresponding to a desirably specified film thickness, a predetermined rotation time and a product of the setting rotation speed and the predetermined rotation time. In the drying step, the square-like (square or rectangular) substrate is rotated at the rotation speed lower than the setting rotation speed in the uniforming step, and thereby, the resist film is dried.
Regarding the above mentioned resist-coating method, in consideration of the film thickness and the concentration (viscosity) of the resist solution, a setting value R of the rotation speed of the square-like (square or rectangular) substrate is specified to be a predetermined value (desirably 250 to 2,000 rpm) within the range of 100 to 6,000 rpm in the uniforming step, a rotation time T is specified to be 20 seconds or less after the rotation speed has reached the setting value R (the predetermined value), and the product (R×T) of the setting value R of the rotation speed and the rotation time T is specified to be 24,000 (rpm×sec) or less. The rotation speed is experimentally specified to be 130 rpm or less in the drying step following the uniforming step. Furthermore, the vapor pressure (at 20° C.) of a solvent is specified to be 20 mmHg or less, while the solvent adjusts the viscosity of the resist solution. By satisfying these conditions, protuberance (fringe: one or various light or dark bands produced by the interference of light due to the resist film thickness variation) of the resist film can be reduced outside the inscribed circle of the square-like (square or rectangular) substrate, and the region of uniform film thickness can be expanded by reduction of this fringe.
However, the above mentioned resist-coating method merely limits the range of the combination (rotation condition) of the rotation speed and the rotation time in the uniforming step to the range inside the curve Ca (the range diagonally shaded with solid lines) in FIG. 2, or merely limits to the desirable range diagonally shaded with broken lines. Consequently, the above mentioned resist-coating method does not propose a technique for selecting the rotation condition to achieve maximum (the best) film thickness uniformity within an effective region (critical area) under the condition practically required in the above mentioned field of application, that is, with respect to a predetermined resist species, a desirably specified film thickness and a desirably specified effective region.
The above mentioned resist-coating method merely specifies an upper limit with respect to selection of the rotation speed in the drying step as well. Therefore, the above mentioned resist-coating method does not propose a technique for selecting the rotation condition to achieve maximum (the best) film thickness uniformity within an effective region nor a technique for optimizing the drying rotation speed.
Furthermore, the above mentioned resist-coating method merely specifies an upper limit of the vapor pressure of the solvent with respect to a method for adjusting the concentration (viscosity) of the resist solution, and therefore, does not propose a technique for optimizing the concentration of the resist solution.
For the reasons described above, conventionally, resist-coating conditions were determined by trial and error within the range of the values of the above mentioned resist-coating method, and therefore, much time was required for achieving desirably specified film thickness uniformity within an effective region.
In the conventional methods, a relative evaluation was able to be performed between individual coating conditions with respect to the film thickness uniformity within an effective region. However, it was not possible to judge whether the determined condition (the rotation speed and the rotation time) is a limit value for the film thickness uniformity within an effective region (that is, the optimum coating condition for achieving maximum film thickness uniformity within an effective region). That is, conventional methods were not able to perform an absolute evaluation of the film thickness uniformity within an effective region.
The resist concentration may significantly fluctuates relative to a specification value or a target value due to variations in the manufacture (variations in quality) of a resist solution (a coating solution) or variations in dilution for adjusting a viscosity. When the resist solution is applied while the resist concentration significantly fluctuates, desirably specified film thickness uniformity within an effective region cannot be achieved.
Regarding some resist species, the film thickness uniformity within an effective region is highly sensitive to the coating condition (the rotation speed and the rotation time) and the coating environment (ambient temperature and humidity), and therefore, desirably specified film thickness uniformity within an effective region cannot be achieved when these coating conditions fluctuate relative to the setting values due to coating device factors and environmental factors.
Regarding some resist species, in order to achieve desirably specified film thickness uniformity within an effective region in a wide range of, for example, 3,000 to 5,000 angstroms or 3,000 to 4,000 angstroms, conventional methods took much time because a coating condition was independently determined with respect to each resist film thickness. In addition, many resist solutions having different concentrations (viscosities) had to be prepared, and a specific facility (a resist solution dispensing device including a filtration mechanism for the solution) was required for each solution.